Many wireless devices, such as cellular telephones, pagers, remote control devices, and the like, are required to operate in multiple RF bands. Examples of wireless devices that are required to operate in multiple RF bands include wireless devices that are to communicate via the 802.11b/g and 802.11a standards, which require communications in the 2.4 GHz band and the 5.2 and 5.8 GHz bands, respectively. Designers of wireless devices, particularly portable wireless devices such as cellular telephones, pagers, remote controllers, and the like, desire and even require antennas that operate in multiple RF bands and that also minimize physical size and fabrication cost. Several types of antennas are incorporated into wireless communications devices, including balanced antennas and unbalanced antennas.
A typical balanced antenna, such as a dipole or a loop, generally requires considerable size or volume within a wireless device. Such antennas can be integrated into the Printed Circuit Board (PCB) of the wireless device, but their size makes their use unattractive or even impractical.
Unbalanced antennas, such as an inverted-F antenna, are generally smaller than conventional balanced antenna structures. However, unbalanced antennas have a significant component of their radiating currents flowing through the ground plane of their wireless device, and are therefore sensitive to perturbations in the wireless device's ground plane. This effect is especially important for personal wireless devices, such as cell phones, that are sometimes, but not always, held in the hand of a user. A personal wireless device, such as a cell phone, has a much different ground plane characteristic when it is far from a person than when it is held in close proximity to a person, such as by a user. A further disadvantage in the use of unbalanced antennas is that many RF circuits used to drive antennas perform better with balanced interfaces to the antenna. An example of such better performance includes suppression of even order harmonics in power amplifiers that are driving a balanced load.
Therefore a need exists to develop an antenna that operates over multiple RF bands and that is particularly suitable for use with portable wireless devices.